ARM: STi: DT: add keyscan for stih416

Add keyscan support for stih416.
It is disabled by default given that it is not enabled on all boards.
Also there are PIOs conflict with already claimed lines.

Acked-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Giuseppe Condorelli <giuseppe.condorelli@st.com>
Signed-off-by: Gabriel Fernandez <gabriel.fernandez@linaro.org>
Signed-off-by: Maxime Coquelin <maxime.coquelin@st.com>